In balanced communication systems, serial data transmission takes place over communication channels in differential mode, and each channel constitutes a complementary pair of transmission lines. It will be appreciated by those skilled in the art that one of the transmission lines is positive, and the other one is negative. Each transmitted bit of data is represented by a differential signal appearing between the two lines, the difference being between a first signal transmitted over positive transmission line and the complement which is transmitted over the negative line of the pair.
Such communication systems often employ numerous communication channels, and the physical routing of cables in such systems often creates an amalgam of intertwined transmission lines. An intrinsic capacitive coupling is formed between every pair of proximate transmission lines, and in a balanced system these intrinsic capacitive couplings often cause noise known as "cross-talk."
Referring to FIG. 1, the incidence of cross-talk may be explained with reference to a generic two-channel balanced communication system. The system includes a first channel CH1 formed by a pair of differential transmission lines positive A and negative A. Likewise, the system includes a second channel CH2 formed by two differential transmission lines positive B and negative B. Intrinsic capacitive couplings exist between all of the transmission lines A, A, B, and B as a result of their physical proximity. Specifically, the intrinsic capacitive couplings may be represented by four capacitances occurring between the two data channels, including C.sub.AB, C.sub.AB, C.sub.BA, and C.sub.BA, respectively. Whenever data transmission is completed via any of the channels, for example, channel 2 (via lines B and B), leakage of the transmitted signal will occur through capacitive couplings C.sub.AB, C.sub.AB, C.sub.BA, and C.sub.BA, and problematic cross-talk signals will appear on the other two transmission lines A and A.
Since the physical proximity of the transmission lines A, A, B, and B generally varies relative to each other, the magnitude of the capacitive couplings C.sub.AB, C.sub.AB, C.sub.BA, and C.sub.BA will be non-uniform. Consequently, the relative magnitudes of the cross-talk signals will vary from line to line.
If the relative magnitudes of the cross-talk signals appearing on the two lines of a single channel are sufficiently different, then the cross-talk signals may be mistaken for differential data transmission.
As an example, we may consider a differential signal as shown in FIG. 2 which is transmitted on channel 2, the positive component being applied to transmission line B and the negative component being applied to line B.
We will assume non-uniform capacitive couplings with relative magnitudes as follows: ##EQU1##
The above-described relations between the capacitive couplings results in four leakage components with exemplary magnitudes as follows.
As shown in FIG. 3, leakage occurs from transmission line B through the capacitive coupling C.sub.AB and to line A to inject a 0 to +1 V cross-talk signal on line A. Further leakage occurs from transmission line B through the capacitive coupling C.sub.AB and to line A to impart a 0 to -0.5 V cross-talk signal on line A. These two leakage components sum to yield a 0 to +0.5 V net cross-talk signal on line A.
Similarly, leakage occurs from transmission line B through the capacitive coupling C.sub.BA and to line A to impart a 0 to +0.5 V cross-talk signal on line A, and leakage occurs from transmission line B through the capacitive coupling C.sub.BA and to line A to impart a 0 to -0.1 V cross-talk signal on line A. These two leakage components sum to yield a 0 to -0.5 V net cross-talk signal on line A. This results in a 1 V differential cross-talk signal across channel 1 which may be mistaken for data transmission.
It would be greatly advantageous if the above-described cross-talk could be reduced or eliminated.